Split the current GC events into generations and into seq/par
(if RequestParGC is not enough to tell seq/par). We don't want to report
the CPU time, only the elapsed time, and that's fine.

Parallel GC work balance: 1.00 (6391526 / 6375794, ideal 2)

This is quite convoluted: total words copied during parallel GCs divided
by the average over all parallel GCs of maximal number of words copied
by any thread in a single par GC.Revents needed: the events added above suffice,
but quite a bit of extra state will have to be maintained when reading
the events. Perhaps +RTS -s could be modified to make this figure simpler?